The present invention relates to a power steering apparatus of hydraulic type that comprises a control valve for supplying hydraulic oil from a hydraulic pump that is driven by an electric motor to a hydraulic cylinder for assisting steering.
A hydraulic type power steering apparatus is arranged in that it comprises a hydraulic cylinder (power cylinder) that is disposed in a steering mechanism of a vehicle to serve as a generating means of steer-assisting force and a hydraulic control valve for performing supply and drainage of hydraulic oil in accordance with operations of a steering wheel, wherein hydraulic oil that is supplied from the hydraulic pump that serves as a hydraulic oil source to the hydraulic control valve is further transferred to the hydraulic cylinder in accordance with operations of the steering wheel to apply hydraulic force (steer-assisting force) that is generated by the hydraulic cylinder in response to the transmission to the steering mechanism, as a result labor and burden of a driver required for steering decrease.
A commonly utilized hydraulic control valve is a valve of rotational type having a valve body and a valve spool that are fitted to each other such that a relative angular displacement is generated in accordance with torsion in a torsion bar that connects an input shaft connected to the steering wheel to an output shaft connected to the steering mechanism. In this arrangement, the valve spool is integrally formed will either one of the input shaft and the output shaft at a connecting portion between the input shaft and the output shaft connected by the torsion bar and value body is connected to the other, such that relative angular displacement according to torsion in the torsion bar are generated between the valve body and the valve spool.
An inner peripheral surface of the valve body is provided with a plurality of first oil grooves at equal intervals which extend in a longitudinal direction, and an outer peripheral surface of the valve spool is provided with a plurality of second oil grooves which are arranged in zigzag positions with the first oil grooves. Throttles are formed between the first and second oil grooves that are adjacent to each other in a peripheral direction, for details, formed between groove edges on each sides in a width direction of the respective oil grooves. The second oil grooves work as oil supply chambers and oil drain chambers at intervals of one. The first oil grooves work on oil transfer chambers between the oil supply chambers and oil drain chambers. Note that the oil supply chambers are communicated to a discharge portion of the hydraulic pump that serves as the hydraulic source, the oil drain chambers are communicated to an oil tank that serves as a destination of drained oil, and the oil transfer chambers are alternately communicated to both cylinder chambers of the hydraulic cylinder that serves as a destination of transferred oil.
In the above arrangement, when input torque (steering torque) is applied to the steering wheel, relative angular displacement is generated in accordance with torsion in the torsion bar between the input shaft and the output shaft, that is, the valve body and the valve spool, so that throttle areas of the throttles are varied. Consequently, hydraulic oil that is supplied from the hydraulic pump is supplied by passing through the throttles with increased throttle areas to the adjoining oil transfer chambers on the same side of the throttles. The hydraulic cylinder then generates hydraulic force according to pressure differences that are generated between the oil chambers on the other side so that the hydraulic force is applied to the steering mechanism as steer-assisting force.
A power steering apparatus comprising a hydraulic control valve performing the above operations is disclosed, for instance, in Japanese Patent Application Laid-Open No. 6-206554 (1994). FIG. 1 is a view showing characteristics of open of throttles and hydraulic force with respect to steering torque of the power steering apparatus. The longitudinal axis represents hydraulic force and throttle open and the lateral axis represents steering torque.
In this power steering apparatus, an electric motor is not driven in a dead zone, namely, until the throttles are completely closed so that no steer-assisting force is generated; however, the electric motor is started driving from the point the throttles are completely closed, and thereafter, control of revolutions is performed in proportion as steering torque is varied, so that hydraulic force can increase in proportion as steering torque increases.
However, in such a conventional power steering apparatus, a corresponding relationship (assisting force characteristics) between steering torque that is applied to the steering wheel and the steer-assisting force that is obtained in accordance with the steering torque therewith is indiscriminately determined by a spring coefficient (torsion characteristics) of the torsion bar for connecting the input shaft and the output shaft. Further, while the assisting force characteristics such that the steer-assisting force increases in proportion to increase in steering torque are obtained, such characteristics are not favorable in performing steering of a vehicle.
That is, steering of a vehicle is performed against reaction force of a road surface acting on wheels for steering (which are generally front wheels), and the degree of reaction force of the road surface is dependent on delay in vehicle speed as well as degree of steering angle, respectively. Due to this reason, when the spring coefficient of the torsion bar is selected to be small with a reference being a large amount of steering at the time of suspension or at the time of low speed running, it is presented a drawback in that steering is performed at a slight force applied to the steering wheel during high speed running to affect stability in straight-ahead running. On the other hand, when the spring coefficient of the torsion bar is selected to be large with a reference being a small amount of steering during high speed running, it is presented a drawback in that satisfactory steer-assisting force can not be obtained at the time of performing steering during suspension, so-called placed-turning, for which a large force is required.
In order to eliminate such drawbacks, the applicants are developing a power steering apparatus that employs a steering angle sensor for detecting a steering angle of a steering means such as a steering wheel, wherein a steering angular velocity of the steering means is obtained by using the detected steering angle, and when the steering angle is equal to or smaller than a predetermined value, the revolutions of the electric motor is made to be equal to or smaller than a predetermined value whereas when the steering angle exceeds the predetermined value, the revolutions of the electric motor is controlled in accordance with the steering angular velocity of the steering means.
However, in such a power steering apparatus that is arranged in that the revolutions of the electric motor is made not to exceeds the predetermined value when the steering angle is equal to or smaller than the predetermined value, and the revolutions of the electric motor is controlled in accordance with the steering angular velocity of the steering means when the steering angle has exceeded the predetermined value, it was presented a drawback in that in case the predetermined value for the steering angle set in the steering angle sensor is set to be approximately 1.degree. to 2.degree. and thus small, the electric motor is driven every time the steering means is steered by approximately 1.degree. to 2.degree. and thus frequently, to result in generation of steer-assisting force. Therefore, it is preferable that the predetermined value for the steering angle be set to be large to approximately 3.degree. to secure proper play such that the electric motor can be prevented from being driven frequently.
FIG. 2 is a hydraulic characteristics view showing a relationship between input torque that is applied to the steering means and hydraulic force that is controlled by the control valve. The longitudinal axis represents hydraulic force and the lateral axis input torque. As it is evident from FIG. 2, the degree of increase in input torque becomes smaller accompanying the increase in hydraulic force that is controlled by the control valve.
However, in case the predetermined value for the steering angle is set to be large to approximately 3.degree., it may be that the driver feels a sense of being tensioned during steering when the steering angle has once exceeded the predetermined value from a steering neutral point of the steering means.
Upon pursuing for reasons causing this sense of being tensioned, it has been found the sense of being tensioned during steering was caused owing to become in discontinuous input torque in C in FIG. 2 due to the following reasons: During a period until the steering angle reaches the set predetermined value .theta. in FIG. 2, relative angular displacements are caused for the valve body and the valve spool of the control valve in manual condition and the pressure of hydraulic oil that is supplied to the hydraulic cylinder is in a slowly rising condition, for example, shows the characteristics at a flow rate of 1.0 l/min of supplied hydraulic oil (a in FIG. 2). When the steering angle has exceeded the set predetermined value .theta., the electric motor is controlled in accordance with the steering angular velocity and hydraulic oil is abruptly supplied from the hydraulic pump to the control valve that is already in a condition in which the pressure of the hydraulic oil can be rising. As a consequence, the condition of the control value shifts to a condition in the characteristics at a flow rate of 2.0 l/min of supplied hydraulic oil. (b in FIG. 2)